U.S. patent application Ser. No. 843,676 filed Oct. 19, 1977, discloses a vaporization cooled transformer design for transformers rated at 3750 kVA and having a basic insulation level (BIL) of 95 kV. The coolant employed within the described vaporization cooled transformer comprises a chlorinated fluorocarbon having a dielectric strength in its liquid state approximately equivalent to transformer mineral oil. When the transformer is operated at rated loading, the vaporized coolant exhibits a sufficiently high pressure to prevent dielectric breakdown from occurring within the winding and between the winding and the transformer wall. When the transformer is operated under conditions of low ambient temperatures, the pressure exhibited by the vaporized coolant is not sufficient to provide the necessary dielectric strength. In order to prevent breakdown from occurring at low ambient conditions, a large amount of solid insulation must be employed around the winding and the spacing between the internal electrical conductors must be large enough to ensure that breakdown does not occur.
When basic insulation levels higher than 95 kV are desired the added insulation material required to compensate for the decreased dielectric strength occurring at low coolant pressures makes the overall transformer size economically infeasible. The larger transformer tank needed to provide the necessary separation distances between the windings and the tank wall, requires a larger quantity of liquid coolant within the tank. The design of a vaporization cooled transformer for operating at higher ratings than 3750 kVA requires a large heat exchanger to cool the coolant in order to prevent the transformer core and windings from becoming overheated at the higher operating temperatures. Since the cooling facility of the heat exchanger depends upon the surface area exposed to the ambient air, a larger number of condenser tubes and auxiliary cooling fans must be employed.
The purpose of this invention is to provide a vaporization cooled transformer having increased voltage ratings without requiring a substantial increase in the overall size of the transformer tank and the heat exchanger.